For economic and environmental reasons, internal combustion engines operated using gas offer an alternative to conventional internal combustion engines operated using gasoline or diesel. Bivalent internal combustion engines that can be operated using both gas and, for example, gasoline are also employed for enhancing the user's flexibility.
Gas-operated systems usually have a tank for storing the gaseous fuel. It is stored under high pressure. For vehicles operated using natural gas the pressure during storage is around 200 bar; said pressure can in the case of hydrogen-operated vehicles assume values of up to approximately 700 bar. The gaseous fuel reaches a gas-injection system via shutoff valves and pressure reducers. The internal combustion engine is finally supplied with gas by means of gas-injection valves assigned to the gas-injection system. The pressure in the gas-injection system is usually constant and for vehicles operated using natural gas is in a range between 3 bar and 15 bar. There are disadvantages in operating the gas-injection system at a constant pressure in that it has to be selected such as to insure the internal combustion engine's reliable operation under any conditions occurring.
Thus when the internal combustion engine is operating under a heavy load, high pressures will be desirable in the gas-injection system to enable a high volume flow through the gas-injection valves. Conversely, when the internal combustion engine is operating under a light load, for example while idling, low pressures will be desirable because its gas supply can be controlled more precisely at low pressures. A low pressure is furthermore advantageous when the internal combustion engine is shut off or, in the case of a bivalent system, is switched over to another type of fuel because the amount of residual gas in the gas-injection system reduces as the pressure falls. The amount of gas that can escape through leaking valves into the surrounding area or into an inlet pipe will thus also reduce. Since the gas-injection valves do not close completely even in their closed position, a certain amount of gas will reach the internal combustion engine via the gas-injection valves even after it has been shut off. Said amount of gas increases in line with the pressure prevailing in the gas-injection system. Said amount of gas will when the internal combustion engine is next started result in undesired enriching of the fuel mixture and hence in increased noxious emissions. More gas can furthermore be taken from the tank owing to the lower pressure and the range of a vehicle being driven by means of the internal combustion engine hence increased.
DE 103 35 152 A1 describes a monitoring device and an operating method for a gas-operated internal combustion engine. For monitoring a gas-operated internal combustion engine a loss of gas leaked from an gas-injection system is determined by means of a measuring device. An operating variable of the internal combustion engine such as, for example, the amount of fuel injected by the gas-injection system as part of a starting process, is then influenced as a function of the determined leaked loss of gas.